The invention relates to a load-introducing armature adapted to be embedded between layers of fibre-reinforced plastic matrix to form a laminated structural element.
It is the purpose of the invention to improve the introduction of load into fibre-reinforced plastic structures. Such plastic structures may be manufactured to have very high weight-specific strength values and/or rigidity values (E-module values). In many cases difficulties arise in constructing and manufacturing a load-introducing armature which by exploiting the high weight-specific strength value of the fibre composition yields a satisfactory result as far as weight and strength is concerned. A typical example is the wing spar of an aeroplane where the heavy loads substantially comprise pressure or tensile loads in the main direction of the fibre-reinforcement.
In such wing spars the load-introducing means comprise a number of thin plates, preferably of metal such as steel/aluminum intercalated between layers of laminate. As the glueing capacity between plastic laminate and plain metal surfaces often is insecure and always difficult to verify and control it is the purpose of the invention to improve the cohesion between armature plates and adjacent layers of fibre-reinforced plastic matrix, the invention being characterized in that the portion of each armature embedded between plastic layers is provided with a plurality of holes having larger opening width than depth. Through these holes plastic matrix layers on either side of the armature plate are mutually connected by pegs consisting of or amalgamatable with the matrix material and completely filling the holes in the armature. When the armature is exposed to load these pegs are subject to shearing stress. Due to the large opening width of the holes in relation to their depth the cross-sectional area of the pegs when subject to shearing stress will be very large, both ends of each peg carrying load. In this way the shearing load acting on the pegs will be very moderate even near the breaking load of the armature.
The provision of a plurality of holes in each armature plate in accordance with the invention in addition gives the possibility to bring about, by a reduction of the spacing between the holes in the direction away from the point of load-introduction on the armature, a gradual transition between the superior stiffness of the armature (as a rule steel or hard aluminum alloy) and the lower stiffness of the adjacent laminate. By a suitable dimensioning of the total hole area in relation to the surrounding material of the armature it is thus easy to gradually increase the resilience of the metal so that the armature near the transition zone between the armature and the laminate has a stiffness in close agreement with that of the laminate. Thus the armature may be specifically adapted to a certain laminate or vice versa.
The characteristic properties of an armature and a laminated structural element according to the invention will appear from the attached claims.